Re: [PATCH] vfio/pci: Don't export DMABUFs for unmappable BARs

Alex Williamson
2026-04-16T21:48:09Z

On Thu, 16 Apr 2026 19:03:40 +0100
Matt Evans <mattev@meta.com> wrote:

> Hi Leon,
>
> On 16/04/2026 14:14, Leon Romanovsky wrote:
> >
> > On Thu, Apr 16, 2026 at 02:05:30PM +0100, Matt Evans wrote:
> >> Hi Leon,
> >>
> >> On 16/04/2026 09:11, Leon Romanovsky wrote:
> >>>> On Wed, Apr 15, 2026 at 11:16:23AM -0700, Matt Evans wrote:
> >>>> Although vfio_pci_core_feature_dma_buf() validates that both requested
> >>>> DMABUF ranges and the PCI resources being referenced are page-aligned,
> >>>> there may be reasons other than alignment that cause a BAR to be
> >>>> unmappable.
> >>>>
> >>>> Add a check for vdev->bar_mmap_supported[index], similar to the VFIO
> >>>> mmap path.
> >>>>
> >>>> Fixes: 5d74781ebc86c ("vfio/pci: Add dma-buf export support for MMIO regions")
> >>>> Signed-off-by: Matt Evans <mattev@meta.com>
> >>>> ---
> >>>> drivers/vfio/pci/vfio_pci_dmabuf.c | 3 +++
> >>>> 1 file changed, 3 insertions(+)
> >>>>
> >>>> diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
> >>>> index f87fd32e4a01..4ccaf3531e02 100644
> >>>> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
> >>>> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> >>>> @@ -249,6 +249,9 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
> >>>> if (get_dma_buf.region_index >= VFIO_PCI_ROM_REGION_INDEX)
> >>>> return -ENODEV;
> >>>> + if (!vdev->bar_mmap_supported[get_dma_buf.region_index])
> >>>> + return -EINVAL;
> >>>> +
> >>>
> >>> And it looks like AI has valid concern about this line too.
> >>> https://urldefense.com/v3/__https://sashiko.dev/*/patchset/20260415181623.1021090-1-mattev@meta.com__;Iw!!Bt8RZUm9aw!5DxsN8cDUviPIZqEjG0pZ_VYYbl_RdmWucTGdTZ3ZzlVP_Ysb0n7ykr0eXwFXdpuqvZH2FK3$
> >>
> >> Ah, Sashiko has a point, and I think its suggestion of checking lower down
> >> in the default .get_dmabuf_phys (vfio_pci_core_get_dmabuf_phys()) and
> >> preserving driver overrides is decent. Will revisit.
> >>
> >> To your other question:
> >>> I noticed this check in vfio_pci_core_mmap(). Isn't that sufficient?
> >>
> >> The scenario in mind is doing a DMABUF-export for BARs that you haven't
> >> necessarily noticed can't be mmap()ed, and both paths should be checking.
> >
> > I added the validation checks that matter on the kernel side, but mmap is
> > primarily important for callers. What I am missing is an explanation of
> > why the kernel should impose this restriction on itself.
>
> I don't understand your question, really sorry! Can you rephrase it
> please? I want to make sure I answer it fully.
>
> Although mmap() fails for BARs that are unmappable (for whatever
> reason), a DMABUF export for the same ones could in some slim cases
> succeed -- because the checks aren't identical. If export succeeds, it
> could potentially allow P2P (or CPU via a future DMABUF mmap()) access
> to something possibly unmappable, no?
>
> For the checks that vfio_pci_probe_mmaps() does (leading to
> bar_mmap_supported[] = false), most have corresponding-but-different
> checks reachable from DMABUF export:
>
> If a BAR is: Then DMABUF export...:
>
> size < pagesize vfio_pci_core_fill_phys_vec() catches it
> Not IORESOURCE_MEM pcim_p2pdma_provider() rejects it
> non_mappable_bars ... nothing? Export allowed
>
> As a quick test, if I hack in non_mappable_bars=1 for my function, it
> appears exporting a DMABUF from it works.
>
> We could add another check for non_mappable_bars, but my thinking was
> that we don't want to keep adding to an independent set of DMABUF
> checks, especially if a future quirk/etc. could create another scenario
> where BARs aren't mappable. I.e. we should reject DMABUF export in
> exactly the same scenarios as mmap() would be rejected, symmetrically,
> by testing bar_mmap_supported[].
>
> Hope that goes some way to answering the Q, hopefully I haven't missed
> something!

That's the concern as I see it as well, it's a choice whether to
attempt to support sub-PAGE_SIZE mappings, but if a device is reporting
non_mappable_bars are we're exporting those BARs through dma-buf for
mmap, that's a problem. Should pcim_p2pdma_provider() test this flag
rather than vfio_pci_dmabuf though? Thanks,

Alex